1.) The phosphate buffer system is very important for maintaining the pH of the cytoplasm of all cells. Phosphoric acid is a triprotic acid; however, the relevant equilibrium in the biologically useful, neutral range, with pKa of 6.86 , is that of dihydrogen phosphate and monophosphate ions (data from Table 2.6)
Reactants | Products | pKa |
---|---|---|
2.14 | ||
6.86 | ||
12.4 |
Using the Henderson-Hasselbalch equation, calculate the pH of a solution containing 0.042 M and 0.058
The acid (HA) in this mixture is , and the conjugate base (A-) is
Substituting these values into the Henderson-Hasselbalch equation:
2.) An undergraduate student is making 300 mL of 1 M potassium phosphate buffer solution with a pH = 6.96 using 1 M and 1 M . How many milliliters of 1 M will be needed to make this solution?
pKa = 7.21
Entering the known values into the Henderson-Hasselbalch equation gives:
So we need to solve for the "mole ratio", aka the part.
Take the anti-log of both sides,
Next we need to find the total mols in solution. It was given as 300 mL of 1 M solution.
Since we are interested in the amount of we will now write the ratio as :
Now we just solve for , notice how we can take out a "1 X" from the right side of the equation.
Finally, we only need to convert mols back into mL using the molarity/concentration provided.